More than 500,000 patients are readmitted to hospital every year soon after being allowed home, figures obtained by the Conservatives show.

The number of patients readmitted as emergencies within 28 days of being discharged rose from 359,719 in 1998-99 to 546,354 in 2007-08, according to the data.

Elderly patients made up a large proportion of those affected - with 159,134 over the age of 75 readmitted in 2007-08, compared with 94,283 in 1998-99.

The Tories said the figures suggest patients are routinely being released before they are well enough.

Shadow health secretary Andrew Lansley said a Conservative government would change NHS rules to stop hospitals being paid again for treating patients they had recently discharged.

He said: “I will ensure that through our payment for results approach, hospitals have to meet any costs arising from emergency readmissions themselves.”

A Department of Health spokesman said: “Patients are only discharged from hospital if the clinicians involved consider it safe and in their best interests.

“Some patients might require readmission if their health deteriorates, but the numbers are small. Only about 5 per cent of patients discharged from hospital are readmitted within seven days of their discharge.

“Rates of readmission can also be a sign of better care. More people with long term conditions are being offered the choice of being treated in the community, with readmission only if the condition worsens.

“A sequence of readmissions can often be preferable to a longer stay in hospital. Treatment in the community is what many people want.”